The present invention relates to a process of treating diamond grains which are used for diamond tools.
In the use of diamond grains, for example for the production of diamond tools such as grinding wheels, dressing or trueing tools for grinding wheels or other diamond application industries such as jewelry or tracing needles provided with diamond, it has been of special importance that diamond grains be sufficiently firmly held in a bonding material in which the grains are embedded.
Bonding materials utilized in diamond tools are metals, metal alloys, glass, ceramics or synthetic resin. Most of these bonding materials are not adhesive to the surface of diamond grains, or adhesive forces are relatively small. Thus, in most cases, the diamond grains are only enveloped by the bonding material. This applies especially to metallic bonds. The exceptions are those metal alloys which contain at least one component which tends to react chemically with the diamond surface or to diffuse into the diamond surface. In the case of such reactive metallic bonding materials, the diamond surface is attacked to a greater or lesser degree, and a part of the diamond carbon is removed by the metal. The resulting roughness produced on the diamond surface can admittedly contribute to an improvement in the anchorage of the diamond grain in the bond. The bonding of the diamond grains into the metallic bond, where such a reaction takes place, occurs at an elevated temperature which is above 900.degree. C. During the subsequent cooling, after the diamond bond has been produced, however, the carbon dissolved or absorbed by the metal is almost always again deposited at the interface between the diamond and the metallic bond, and this deposited carbon has no significant strength. This can be a graphite layer, or a layer of amorphous carbon or brittle carbide. No effective improvement in the adhesive bond is therefore achieved in this way at the interface between the diamond grain and the bond. The deposited carbon layer can even represent a weakening of the bond between the bonding metal and the diamond grains.
It is known that diamond grains are attacked and roughened if they are surrounded by a metal powder matrix in the sintering process, and metal powders utilized consist of iron or cobalt or of alloys of these metals. Incipient etching of diamond surfaces is detectable when the sintered alloys are processed in the range between 950.degree. C. and 1150.degree. C.
In another known process, diamond grains are heated while freely exposed in an oven, the atmosphere of which consists of oxygen or an oxygen-containing gas such as air, for example. In this case, diamond grains are attacked even at about 600.degree. C. and higher. This is thus a slowed-down combustion of the diamond carbon to carbon monoxide CO, the diamond material being removed from the diamond surface, and small etching pits also being formed.
A similar incipient etching of diamond surfaces can be obtained, for example, by treating diamond grains in a melt of potassium nitrate KNO.sub.3 at a temperature of 500.degree. C. and higher.
None of the abovementioned etching processes or roughening processes give fully satisfactory results. On the one hand, this is caused by the reaction temperatures of more than 900.degree. C., where not only the surface of the diamond is attacked, but the internal strength of the crystal is also reduced. This applies particularly to synthetic diamond grains which, in the normal case, still contain traces of the catalyst metal used for their preparation. At temperatures above 900.degree. C., these catalyst residues lead to regraphitization of the diamonds or even to a partial disintegration of the crystal due to the relatively large thermal expansion of these metal residues as compared with that of diamond material.
On the other hand, the essential disadvantage of the known processes is that a relatively large quantity of material must be removed from the diamond crystals before the surface shows a satisfactory roughness, since these roughening or etching processes are surface-wearing processes which do not progress parallel to the surface. This undesired, relatively large removal of material from diamond crystals manifests itself by the fact that, for roughening a diamond grain of about 500 .mu.m grain size, a weight loss of 10% must be accepted, and this already means a considerable loss in value. Various known roughening processes even cause weight losses of about 30%.